The invention is directed to the field of microsensor mass air flow devices and a method of fabricating the devices. The structure at an intermediate point of fabrication is a single crystal silicon substrate chip having on a portion of the surface a thin sacrificial layer of aluminum or other selectively etchable layer which delineates the future location of a diaphragm to be formed at a later step. The sacrificial layer of aluminum and the rest of the silicon chip surface are coated with a thin film of silicon nitride which forms the diaphragm. By sacrificial is meant a layer which is removed at a later stage in the fabrication process.
A microbridge air flow sensor is needed that is not susceptible to contamination from residual films or other material accumulating within the etch pit or cavity which thermally isolates the heater and detectors. It is also needful to fabricate the device by front side etching and to have the silicon chip edges oriented orthogonally to the air flow direction.
In the prior art of sealed-off silicon nitride micro-diaphragms having an etched cavity beneath the diaphragm, there is a publication by S. Sugiyama et al of Toyota Central Research Labs entitled "Micro-Diaphragm Pressure Sensor", IEDM 1986, pages 184-187. In that disclosure a micro-diaphragm pressure sensor with silicon nitride diaphragm of 80 micron.times.80 micron was fabricated by applying micromachining technique. A main feature was said to be that a planar type pressure sensor was formed by single-side processing solely on the top surface of (100) silicon wafer. The diaphragm and a reference pressure chamber are formed by undercut-etching of the interface between the diaphragm and the silicon substrate. A 1500.ANG. thick interlayer, such as polysilicon, is formed on the interface, that is, on the silicon surface and under the diaphragm. An etch. hole(s) is/are opened in the diaphragm to the polysilicon and an anisotropic etchant KOH is used to remove the polysilicon interlayer and to anisotropically etch the silicon substrate under the diaphragm.
Although there are similarities between the Sugiyama pressure sensor diaphragm structure and the flow sensor structure of the present invention, there are nevertheless major differences between them such that the present invention could not be fabricated by the method taught by Sugiyama. They are similar in that both structures make use of a layer of a sacrificial material that is etched away in the fabrication of the device, both use front surface etching, and both have an anisotropically etched pit in the silicon beneath the diaphragm containing the sensing resistors.
One of the significant differences is that the Sugiyama etch pit has about 6 percent of the surface area and about one to two percent of the volume of the pit of the subject invention. Furthermore the Sugiyama diaphragm has no heater and need not be heated, whereas the subject invention flow sensor diaphragm has an embedded heater element and must be heated, typically to 160.degree. C. above the silicon chip ambient temperature.
Thirdly, the Sugiyama pressure sensor is fabricated with a single etchant and a single etch step to remove both the polysilicon sacrificial layer and the silicon from the pit volume to give a pyramidal shaped pit. As will be described in detail, the subject invention is fabricated using one acid etchant to selectively remove only a metallic sacrificial layer, followed by a different alkaline anisotropic etchant to selectively remove only the silicon pit material. This enables the accurate fabrication of a large, shallow, flat bottomed pit without penetrating all the way through the thin 0.010" wafer required for the flow sensor. Such a pit is not obtainable by the Sugiyama technique in which the resulting pyramidal pit would penetrate all the way through the thin wafer when making the large area pit of the subject invention. Penetration through the entire thickness of the wafer is unacceptable because the chip is weakened, and because the epoxy or solder used to cement the chip to its package can then well up into the pit and change the thermal conductance in an unpredictable way.
The Sugiyama pressure sensor requires a permanently evacuated cavity and therefore requires a hermetic seal which is obtained by plasma depositing an impermeable silicon nitride layer at very low pressure to seal a narrow etch channel that is only 0.15 microns wide. On the other hand, the subject invention requires a cavity sealed only to the extent that dust and liquids are excluded, and it is necessary to have the pressure within the pit approximately atmospheric to avoid rupturing the diaphragm and changing the thermal efficiency of the heated diaphragm significantly.
The narrow 0.15 micron wide etch channels of Sugiyama would not allow enough access for the etchant to etch out the large cavity of the subject invention in a practical length of time. Therefore the subject invention uses an array of slots in the silicon nitride diaphragm, each slot being typically 2.0 microns wide and 5 to 5 microns long and only about one micron deep. This array of slots provides adequate access for the sequentially used etchants to remove the sacrificial metallic layer and to etch out the pit in a practical length of time.
The 2.0 micron wide slots of the present invention access the pit vertically through the approximately one micron thickness of the diaphragm nitride, whereas the 0.15 micron wide channels of the Sugiyama device extend horizontally over several microns to reach the cavity. The Sugiyama 0.15 micron channels are sealed off by building up vertically a layer of nitride across the outlet of the channel. In this process the rate of closure of the channel is constant because the nitride molecules always have direct access to the horizontal deposition surfaces. However, with the Sugiyama method of plasma deposition of silicon nitride, it is impossible to seal off the slots of the subject invention that are 2.0 microns wide in a practical length of deposition time. This is because the plasma nitride accumulates even more slowly on the vertically oriented sides of the slots as they close, the nitride molecules tending to come to rest on the outer, more horizontal surfaces before reaching the inner closing surfaces. Therefore, in the subject invention, the slots are sealed with a thin layer of a viscous polyimide solution which is flowed rapidly over the surface, then hard-baked to form a mechanically tough, but not absolutely hermetic layer on the diaphragm. This layer can be left intact, or can be delineated to leave polyimide covering only the local areas of the slots.
Thus, because of the relatively larger size of the subject invention and the different cavity shape that is required, a uniquely different structure of the diaphragm and a uniquely different fabrication method is required.